This invention relates to a process for preparing low molecular weight phosphonate-terminated polymers. In particular, this invention relates to a process for the efficient use of phosphorous acid as a chain transfer agent in aqueous polymerizations.
Low molecular weight polycarboxylic acid polymers and their salts are useful as dispersants, scale inhibitors, detergent additives, sequestrants, etc. Generally a molecular weight below 50,000 is necessary for effective performance and often very low molecular weights below 10,000 are most effective. It is common to use chain transfer agents in the polymerization reaction to produce low, and especially the very low, molecular weight polymers. Phosphorous and hypophosphorous acid and their corresponding salts (commonly sodium phosphite and hypophosphite, respectively) are particularly desirable chain transfer agents, chosen primarily because they introduce phosphonate and phosphinate functionality, respectively, into water-soluble polymer molecules which confers superior performance properties in some applications. As used hereinafter and in the appended claims, the term "phosphorous acid" is intended to include phosphorous acid and its derivatives, for example, salts thereof, such as alkali and alkaline earth metal salts (metal phosphites), unless a clearly different meaning is indicated.
Inefficiency is a problem that is common to most of the known processes utilizing phosphorous acid as a chain transfer agent. A significant portion is not incorporated into the polymer and remains unreacted or is converted to other species such as phosphoric acid or corresponding salts. As a consequence, high levels of phosphorous acid are required to obtain low molecular weight polymer. Since phosphorous acid is relatively costly its use may be prohibitively expensive if the phosphorous acid is not used efficiently.
A second disadvantage arising from the inefficiency is the significant amounts of unreacted phosphite or oxidized byproduct, such as phosphate, salt residues present in the reaction product. These salts do not contribute to performance thereby diluting the activity of the reaction product. In some cases, such as preparing concentrated clay slurries, these salts can interfere with the dispersing process.
Low molecular weight polycarboxylic acid polymers are conventionally produced by reacting certain olefinic monomers with phosphorous acid or phosphorous acid derivatives in the presence of a free-radical initiator. Yields of the desired phosphonate-terminated polymer are low, and the polymer contains high levels of residual phosphorous acid and phosphate impurities. When derivatives of phosphorous acid, for example esters, are used as chain transfer agents, separate hydrolysis and stripping steps are required to recover the polymer. For example, EP 360746A discloses a method of preparing copolymers containing phosphonate endgroups involving the reaction of olefinic reactants with phosphorous acid or derivatives thereof (such as dialkyl phosphites or alkali metal salts) in the presence of a free-radical initiator. The use of dialkyl phosphites as starting materials in this method requires the use of non-aqueous systems and subsequent hydrolysis and purification.
Another method of preparing copolymers containing phosphonate endgroups using hypophosphorous acid is disclosed in U.S. Pat. No. 5,376,731. The method disclosed involves the reaction of an olefinic reactant with a monoalkyl phosphinate to form a dialkyl phosphinate intermediate, followed by oxidation of the latter to a phosphonate-terminated polymer. Hypophosphorous acid is initially reacted with a carbonyl compound to form the monoalkyl phosphinate. This process involves several separate steps, that may require isolation of intermediates, etc., and has the further disadvantage of producing a mixture of phosphinate and phosphonate-terminated polymer product, unreacted hypophosphite and chloride byproducts.
U.S. Pat. No. 5,386,038 discloses the preparation of phosphonate-terminated oligomers of maleic acid and acrylic acid using sodium phosphite in the presence of a high levels of free-radical initiator. U.S. Pat. No. 4,046,707 discloses the preparation of copolymers containing phosphonate and phosphinate endgroups involving the reaction of olefinic reactants with phosphorous acid or hypophosphorous acid and derivatives thereof (such as dialkyl phosphites, alkyl hypophosphites, phosphinic acids and alkali metal salts) in the presence of a free-radical initiator; the use of phosphorous acid to prepare low molecular weight poly(acrylic acid) is disclosed. GB 996,737 discloses the use of phosphorous acid or its salts to produce high molecular weight polymers that are useful in producing fibers, where the pH of the reaction mixture is adjusted to 4-5 before the polymerization. U.S. Pat. No. 4,621,127 discloses the preparation of carboxyl-containing polymers where inorganic phosphorus compounds, including hypophosphite and phosphite, are used to improve the color of the resulting polymer solutions.
The present invention seeks to overcome the problems associated with prior art processes used to prepare phosphonate-terminated polymers.